Proteins

Proteins R O | H || —C— C—OH N H Monomer: Amino Acid Atoms: carbon, hydrogen, oxygen, nitrogen amino = NH2 acid = COOH Monomer: Amino Acid R C—OH || O | —C— H N H *20 Different types of amino acids

Nonpolar side chains; hydrophobic ) Glycine (Gly or G) Alanine (Ala or A) Valine (Val or V) Leucine (Leu or L) Isoleucine (le or ) Figure 3.17a The 20 amino acids of proteins (part 1: nonpolar) Methionine (Met or M) Phenylalanine (Phe or F) Tryptophan (Trp or W) Proline (Pro or P) 3

Polar side chains; hydrophilic Serine (Ser or S) Threonine (Thr or T) Cysteine (Cys or C) Figure 3.17b The 20 amino acids of proteins (part 2: polar) Tyrosine (Tyr or Y) Asparagine (Asn or N) Glutamine (Gln or Q) 4

Electrically charged side chains; hydrophilic Basic (positively charged) Acidic (negatively charged) Figure 3.17c The 20 amino acids of proteins (part 3: charged) Aspartic acid (Asp or D) Glutamic acid (Glu or E) Lysine (Lys or K) Arginine (Arg or R) Histidine (His or H) 5

Monomer: Polypeptide enzyme

Side chains Back- bone Peptide bond Amino end (N-terminus) Figure 3.18b Making a polypeptide chain (part 2: new peptide bond) Peptide bond Amino end (N-terminus) Carboxyl end (C-terminus) 7

Proteins Most structurally & functionally diverse group Function: involved in almost everything that goes on in a living thing! (50% dry mass of cells enzymes (pepsin, DNA polymerase) structure (keratin, collagen) carriers & transport (hemoglobin, aquaporin) cell communication signals (insulin & other hormones) receptors defense (antibodies) movement (actin & myosin) storage (bean seed proteins) Storage: beans (seed proteins) Movement: muscle fibers Cell surface proteins: labels that ID cell as self vs. foreign Antibodies: recognize the labels ENZYMES!!!!

Protein models Protein structure visualized by X-ray crystallography extrapolating from amino acid sequence computer modelling lysozyme

Protein Structure and Functions *Each protein is made up of one or more polypeptide chains twisted, folded and coiled into a unique shape *4 Levels to protein structure

Primary Structure Sequence (order) of amino acids in chain *This order is determined by DNA instructions *Even just one amino acid change can make a big difference

Hydrophilic! Hydrophobic! Secondary and Tertiary Structures Primary Quaternary Structure Red Blood Cell Shape Function Normal hemoglobin Molecules do not associate with one another; each carries oxygen. 1 2 3 4 Normal  5  subunit  6  7 5 m  Hydrophilic! Exposed hydro- phobic region Sickle-cell hemoglobin Molecules crystallized into a fiber; capacity to carry oxygen is reduced. 1 2 Figure 3.22 A single amino acid substitution in a protein causes sickle-cell disease. 3 Sickle-cell 4  5  6  subunit  7  5 m Hydrophobic! 14

Secondary Structures Coils and folds called Alpha Helix and Beta pleated sheets caused by hydrogen bonding between polypeptide backbones

Tertiary Structure Three dimensional shape caused by “weak” interactions between the side chains Hydrophobic interactions and van der Waals interactions Hydrogen bond Disulfide bridge Ionic bond Figure 3.21d Exploring levels of protein structure (part 8: tertiary stabilization) Polypeptide backbone 16

Quaternary Structure The overall structure formed by two or more polypeptides This is when protein becomes functional Caused by hydrophobic interatctions hemoglobin collagen = skin & tendons

Protein structure (review) R groups hydrophobic interactions disulfide bridges (H & ionic bonds) 3° multiple polypeptides hydrophobic interactions 1° sequence determines structure and… structure determines function. Change the sequence & that changes the structure which changes the function. amino acid sequence peptide bonds 4° 2° determined by DNA R groups H bonds

Chaperonins assist in proper folding of proteins

Protein denaturation Denaturation is the unfolding of a protein Conditions that disrupt H bonds, ionic bonds, disulfide bridges *Temperature *pH *salinity Result in changes in secondary and tertiary structures Wrong shape = no function